A single-component developer including only toner, and a two-component developer including toner and carrier are known as developers for use in electrophotographic apparatuses. In a developing method using the two-component developer, toner is usually charged by agitating the toner and carrier in a developer reservoir so as to cause the toner to adhere to a surface of the carrier. The carrier having the toner adhering thereto is transported by the developer carrier to a development area where the developer carrier faces an electrostatic latent image carrier. Consequently, a sufficient amount of developer is transported, and a high-density image is obtained. However, when the two-component developer is used, in order to maintain the development density at a certain level, it is necessary to keep the mixing ratio of the toner and the carrier uniform, complicating the entire structure of a development apparatus.
Whereas a development apparatus using the single-component developer can be easily handled. A known example of the single-component developer (hereinafter just referred to as the toner) is a nonmagnetic single-component developer. In general, in a development apparatus that does not use a developer supply member for supplying and applying the nonmagnetic toner, the toner is usually transported to an area between the developer carrier and a nip section of a developer regulating member. More specifically, an amount of toner is transported to the area between the developer carrier and the nip section of the developer regulating member by a movement of the developer carrier. The amount of toner transported is decided by image force, mechanical adhesion such as Van der Waals force to the developer carrier, shearing force due to the flowability of the toner, and the shape of the developer carrier.
The developer regulating member is pressed against the developer carrier by a suitable pressure so as to produce an even thin layer of the toner. Accordingly, most of the toner transported by the movement of the developer carrier cannot pass through the developer regulating member.
Whether the transported toner on the developer carrier can pass through the developer regulating member or not (i.e., toner passing rate) is decided as follows. Namely, the toner passing rate is determined by the relation among the adhesion of the toner to the developer carrier, the toner transporting capacity by friction between the toner and the developer carrier, the pressure applied to the developer carrier by the developer regulating member, and friction between the developer regulating member and the toner.
There is a method for improving the toner supply capacity using a developer supply member (for example, a toner supply roller made of a sponge and the like) in a development apparatus. In this method, the supply of the toner is mechanically and electrically performed by pressing the developer supply member against the developer carrier. More specifically, the developer supply member improves the adhesion of the toner to the developer carrier due to the image force by charging the toner in a contact section between the developer supply member and the developer carrier. Moreover, since the developer supply member performs the function of applying the toner to the developer carrier, the toner passing rate is improved, the toner can easily pass through the developer regulating member, and a satisfactory toner supply capacity is achieved. In short, it is possible to supply a sufficient amount of toner.
A typical example of this kind of a development apparatus will be explained with reference to FIG. 13. First, an electrostatic image is formed on a surface of a photoreceptor 71 (electrostatic latent image carrier) by latent image forming means, not shown. Subsequently, the electrostatic image is developed into a visible form by adhering toner 76 (nonmagnetic single-component developer) to the electrostatic image using static electricity by means of a developing roller 72 (developer carrier) facing the photoreceptor 71.
In the development process, the charged toner 76 forms a thin layer on the developing roller 72. The formation of a thin layer of the toner 76 is performed as follows. First, the toner 76 is supplied and applied to the developing roller 72 by a toner supply roller 77 (developer supply member) which is installed in a developer reservoir 75 and pushed against the developing roller 72. Thereafter, the toner 76 adhering to the developing roller 72 is leveled to form a thin layer by a blade 74a as a regulating member of the developer regulating member 74.
The toner 76 is usually charged by friction or injection of charges at least either in a section between the developing roller 72 and the toner supply roller 77 or in a section between the developing roller 72 and the blade 74a.
In the development apparatus using such a nonmagnetic single-component developer, transport of the toner by the carrier is not available. Therefore, when development which consumes a large amount of toner, for example, development of an entirely black-solid document, is performed, it is impossible to supply an amount of toner that compensates for the consumption of toner on the developing roller, causing an increase in the possibility of a gradual lowering of the density.
For example, using magnetic toner as a single-component developer instead of the nonmagnetic toner is a well known prior art for solving this problem. Namely, since the magnetic toner is magnetically attracted by a magnetic force and transported, it is possible to supply to the developing roller a sufficient amount of toner for compensating for the consumption of toner. However, since the magnetic toner is produced by adding magnetic powder to a principal material, it is difficult to color the magnetic toner and hard to correspond to a color image.
Therefore, using the nonmagnetic toner as the single-component developer brings advantage, However, this causes problems in transporting the toner, charging the toner, and leveling the toner into a thin layer on the developer carrier.
Specifically, with the use of the nonmagnetic toner, since the transport of toner using the magnetic force cannot be performed, it is necessary to modify the method for transporting toner. In contrast to the magnetic toner which adheres to the developing roller due to the magnetic force, the adhesion of the nonmagnetic toner to the developing roller is produced mainly by the image force and Van der Waals force. Therefore, if the toner layer is not thin, the toner falls down or flies from the developing roller. Additionally, when the toner is charged by friction between the toner and the developing roller or the blade, the amount of charge is in inverse proportion to the thickness of the toner layer. Thus, in order to achieve stably high charging of toner, it is necessary to form an even thin layer.
As a toner transporting method for a development apparatus using the nonmagnetic toner, a number of techniques have been disclosed as well as the method using the toner supply roller that is described above as an example of the prior art.
For example, Japanese Publication for Unexamined Patent Application (Tokukaisho) No. 58-98762 and U.S. Pat. No. 4,083,326 disclose methods using a fiber brush as the toner supply roller. Namely, toner held by the fiber brush is supplied by bringing the toner supply roller having the fiber brush on a surface thereof into contact with the developing roller. Moreover, Japanese Publication for Unexamined Patent Application (Tokukaihei) No. 2-191974 discloses a method using an elastic foam body as the toner supply roller. In this method, the toner is supplied by arranging an expanded cell of the elastic foam body to contain the toner.
However, when the toner supply roller made of the fiber brush or the elastic foam body is used, it is necessary to push the toner supply roller against the developing roller in order to supply a sufficient amount of toner. Therefore, stress is applied to the toner in the contact section of the toner supply roller and the developing roller, and the toner tends to deteriorate. Another problem is an increase in the torque of the developing roller.
When the fiber brush is used as the toner supply roller, the toner supply roller tends to deteriorate with time, for example, the brush is clogged with the toner or the hair of brush is laid down. When the elastic foam body is used, the toner supply roller also deteriorates with time because the expanded cell is clogged with the toner.
Furthermore, the installation of the toner supply roller and a driving device thereof not only complicates the construction of a development device, but also loses the simplicity of the apparatus that is one of the advantages produced by the use of the single-component developer, and simultaneously increases the cost.